Azeotropic composition



J 2,999,816 Patented Sept. 1 2 1961 2,999,816 AZEOTROPIC COMPOSITION Edward J. Bennett, Newark, Del., and Howard M. Parmelee, Woodstown, N.J., assignors to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Filed Aug. 15, 1960, Ser. No. 49,427 1 Claim. (Cl. 252-171) This invention relates to an azeotropic composition and particularly to the azeotropic mixture of 1,1,2-trichloro- 1,2,2-trifluoroethane and methyl alcohol.

Several of the chloroiluoromethanes and chlorofluoroethanes have attained widespread use as speciality solvents in recent years, particularly trichlorofiuorornethane and trichlorotrifluoroethane. Both of these compounds are relatively low boiling liquids (CClgF, 23.8 C.; CCl FCClF 47.6 C.), which are nontoxic and nonfiammable, and which have satisfactory solvent power for greases, oils, waxes and the like. They have therefore found widespread use for cleaning electric motors, compressors, oxygen storage tanks, photographic film, lithographic plates, typewriters, instruments, gauges, sound tape, and as noncor-rosive brines.

For certain solvent purposes however, the chlorofiuoromethanes and chlorofluoroethanes have insufficient solvent power alone. This is particularly true in the electronic industry during the manufacture of printed circuits. Printed circuits are well known in the electronics art; and consist of a circuit formed from a soft metal on a solid, nonconducting surface such as a reinforced phenolic resin. During manufacture, the solid surface is coated with the metal, the desired portion of metal is coated with an impervious coating, and the excess metal is removed by etching with a suitable acid. After the excess metal has been removed, it is necessary to remove the impervious coating because solder joints must be made to the printed circuit and these will not form if the coating is present. After the impervious coating is removed, the circuits are coated with a rosin flux to permit the joints to be soldered, which rosin flux must then be removed. The chlorofluoromethane and chlorofluoroethane solvents do not have sufficient solvent power to clean printed circuits; that is, to remove the rosin flux.

It is an object of this invention to provide a novel azeotropic composition which has new and unusual properties. Another object is to provide an axeotropic composition which is valuable as a solvent, and particularly for cleaning printed circuits. A further object is to provide such a composition which is nonfiammable and substantially nontoxic, both in the liquid phase and in the vapor phase. Other objects are to provide new compositions of matter and to advance the art. Still other objects will appear hereinafter.

The above and other objects may be accomplished in accord with this invention which comprises an azeotropic composition consisting of about 93.6% by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and about 6.4% by weight of methyl alcohol and boiling at 39 C. at 760 mm. pressure.

It has been found that 1,1,2-trichloro-1,2,2-trifiuoroethane and methyl alcohol, when admixed in the above proportions, form an azeotrope which distils at constant composition, the liquid phase and the vapor phase in equilibrium therewith having the same composition. Such mixture is nonflammable and nontoxic in both the liquid phase and the vapor phase. It is useful as a heat exchange medium and as a hydraulic iluid. It is particularly useful as a solvent for greases, oils, waxes, and the like, in cleaning electric motors, compressors, photographic film, oxygen storage tanks, lithographic plates,

is a poor solvent therefor, it has been foundthat the small proportion of methyl alcohol present in the azeotrope with the 1,1,2-trichloroe1,2,2-trifluoroethane sufiicientlymodifies the solvent properties of both compo.- nents so as to make the azeotrope an excellent solvent mixture for such'purposes.

The azeotrope of this invention has several advantages over the solvents heretofore employed for cleaning printed circuits. Trichloroethylene, tetrachloroethylene, and methyl chloroform, are unstable and require stabilizers, whereas the azeotropic composition of this invention does not. The azeotropic composition of this invention is more'eifective as a solvent for suchpurposes than trichloroethylene and tetrachloroethylene. Inhibited methyl chloroform (containing dioxane as a stabilizer) is too strong a solvent and attacks the backing of the printed circuits. Isopropyl alcohol-toluene blends are flammable and irritating. The azeotropic mixture-of 1,1,2-trichloro- 1,2,2-trifluoroethane and methyl alcohol does not suffer from any of these defects. The small concentration of methyl alcohol in the azeotropic mixture is insufficient to render the mixture materially more toxic than the trichlorotrifluoroethane.

The azeotropic composition of this invention was obtained during the distillation of a mixture of 1780 grams of 1,1,Z-trichloro-1,2,2-trifluoroethane, 140 grams of ethyl alcohol and grams of methyl alcohol through a 1 inch x 38 inch column packed with Ms inch glass helices at a reflux ratio of 10/1 and a take off of 1 ml./min. The first fraction obtained was 994 grams of the azeo trope, containing 93.6% by weight of 1,1,2-trichloro- 1,2,2-trifluoroethane and 6.4% by weight of methyl alcohol and having a boiling point of 39 C. at 760 mm. pressure. Thereafter, there were obtained a fraction consisting of pure 1,1,2-trichloro-1,2,2-trifluoroethane at 42 C. and fractions which are nonazeotropic mixtures of 1,1,2-trich1oro-1,2,2-trifluoroethane with ethyl alcohol and small amounts of methyl alcohol.

The existence of the azeotrope was further confirmed when a mixture, containing 1700 grams of 1,1,2-trichloro-1,2,2-trifluoroethane and 300 grams of methyl alcohol, was distilled through the same column and under the same conditions as above described. The first fraction distilled Was 1792.4 grams of the azeotrope, containing 93.7% by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and 6.3% by weight of methyl alcohol,

having a boiling point of 39 C. at 760* mm. pressure- At this point, insufficient 1,1,2-trichloro-1,2,2trifluor0- ethane remained to maintain the azeotrope, and non azeotropic mixtures of 1,1,Z-trichloro-l,2,2-trifluoroethane, containing higher concentrations of methyl alcohol, distilled.

Printed circuit boards are usually prepared by impregnating glass cloth, nylon, or paper laminates with a phenol-formaldehyde resin or an epoxy resin. Printed circuits are prepared by a variety of methods. In a typical procedure, the board consists originally of a phenolic resin impregnated base to which is bonded a sheet of copper, 2 to 4 mils thick, covering one surface of the board. The desired circuit is drawn on the copper with an asphalt based ink, using the silk screen method. The excess copper is then removed by etching with a ferric chloride-hydrochloric acid bath, sometimes containing ammonium chloride, leaving on the board the copper that is covered by the ink. After washing off the etch solution, the asphalt ink is removed by cleaning with the azeotropic composition of this invention in an ultrasonic bath (some mechanical scrubbing is often used). The entire surface of the board is coated with a rosin flux and dried. The electronic components (resistors, capacitors, etc.) are then added at the proper places for soldering to the circuit. The board is then passed over a molten solder bath, contacting the desired joints with the molten metal, whereby the soldering is effected. After cooling, the excess rosin flux remaining on the board must be removed since, if present in the final assembly, it will lead to corrosion, poor electrical resistance and other deleterious properties.

The board is cleaned by placing it in an ultrasonic bath operating at about 32 kilocycles per second and at about 25 C., where it remains for one minute. If 1,1,2-trichloro-1,2,2-trifluoroethane is used alone as the solvent in the bath, it has no efiect on the rosin flux. If methyl alcohol is used alone as the solvent in the bath, about 40% of the flux is removed. If trichloroethylene is used as the solvent in the bath, about 5% to about 50% of the flux remains after the treatment. If the same cleaning operation is repeated on another circuit board using the azeotropic mixture of 1,1,2-trichloro-l,2,2-tnifluoroethane and methyl alcohol as the solvent, the rosin flux is completely removed. Neither the 1,l,2-trichloro-1,2,2-triflnoroethane, the methyl alcohol, nor the azeotropic mixture thereof has any effect on the board which constitutes the backing of the printed circuit.

It will be understood that the preceding detailed description of the use of the azeotropic composition of this invention in cleaning printed circuits is given for illustrative purposes solely, that this invention is not restricted to such specific embodiment and that other techniques may be employed. Also, the azeotropic composition can be used for other purposes as indicated in the general description.

From the preceding description, it will be apparent that this invention provides a novel azeotropic composition which has unusual properties and which is useful for a wide variety of purposes. Accordingly, it will be apparent that this invention constitutes a valuable contribution to and advance in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

An azeotropic composition consisting of about 93.6% by weight of 1,1,Z-trichloro-1,2,2-tuifluoroethane and about 6.4% by Weight of methyl alcohol and boiling at 39 C. and 760 mm. pressure.

References Cited in the file of this patent Industrial Solvents, Mellan, 2nd ed. (1950), pp. 48- 79, 372-373, 448-453. 

